This invention relates to fluid collection devices, and more particularly, to blood collection devices having means for partitioning the relatively light phase from the relatively heavy phase.
When taking blood samples for test purposes, whole blood is generally drawn into an evacuated collection tube and the tube subsequently centrifuged to separate the blood into its relatively light phase, serum or plasma, and its heavier cellular phase. Blood phase separators or portitioning devices have been used to provide a barrier between the separated phases until the light phase is removed for clinical testing.
Many types of blood phase partitioning devices have been proposed, all with varying degrees of success. A relatively simple partitioning arrangement includes the use of a quantity of gel-like thixotropic material having a specific gravity intermediate the specific gravities of the light and heavy blood phases so that during centrifugation and phase separation, the material automatically flows to the interface of the two phases and forms a partition or barrier between the phases. Various gel-like thixotropic materials or sealants are now well known. For example, in U.S. Pat. No. 3,852,194, a mixture of silicone and hydrophobic silicon dioxide powders is used to form a partition between the separated phases. In U.S. Pat. Nos. 4,021,340; 4,088,582 and 4,055,501, mixtures including liquid polybutene polymer and silicon dioxide powders are used as phase partitioning materials.
A problem in the use of phase partitioning sealants has been that desired flow characteristics have not always been obtained. For example, in some cases the sealant may form a partition too soon and trap blood cells in the light phase, causing contamination. For certain tests, such contamination can result in unreliable or inaccurate test results. On the other hand, in some cases the sealant may remain in its initial location, adhering to the collection tube, and not flow during centrifugation to the interface of the two phases, thus failing to form a partition. Where the relatively small difference in specific gravities of the sealant and heavy phase is relied upon to provide the force necessary to cause the sealant to flow toward the interface, reliable flow characteristics may not always be obtained.
In some arrangements the sealant flows upwardly in the collection tube, interfering with the downward flow of cells or blood clots during centrifugation. This may result in the sealant carrying cells to the interface or trapping them so that they remain in contact with the separated light phase. Also, the impact of cells with the sealant during centrifugation is believed to cause cell homolysis and an artificial increase in lactic dehydrogenase (LDH). Where this occurs, the LDH result is, of course, inaccurate.